Process for preparing a polyurethane foam in the presence of a hydrocarbon blowing agent

ABSTRACT

This disclosure relates to polyisocyanate-based foams, particularly to polyurethane or polyurethane-polyisocyanurate foam prepared in the presence of a certain blowing agent. The blowing agent, based on total amounts of a) and b), consists of: (a) from about 5 to about 80 mole percent of a C 5-6  alicyclic alkane or mixtures of such; and (b) from about 95 to about 20 mole percent of a mixture of isopentane and n-pentane present in a mole ratio of from about 80:20 to about 20:80.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 08/174,555, filed Dec.27, 1993.

BACKGROUND OF THE INVENTION

This invention relates to a polyurethane orpolyurethane-polyisocyanurate foam prepared in the presence of a blowingagent which consists of an alicyclic and an aliphatic alkane.

It is well known to prepare cellular polyurethanes by reacting apolyisocyanate with an active hydrogen-containing material in thepresence of a halocarbon blowing agent. In preparing flexiblepolyurethane foam, for example, halocarbons are often used to supplementwater in generating a gas for blowing. However, the use of halocarbonblowing agents is particularly significant in making rigid foam. Inrigid foam, the halocarbon blowing agent additionally provides thermalinsulating properties and usually constitute the major proportion of theblowing agent when preparing a rigid foam. Of the halocarbons,trichlorofluoromethane (R-11), dichlorodifluoromethane (R-12) andtrichlorotrifluoroethane (R-113) are of the most commercial importance;due to their advantageous thermal insulating properties, boiling pointsand stability. A disadvantage of these substances is that they arehighly suspected of contributing to the depletion of atmospheric ozone.Accordingly, it would be highly desirable to replace the halocarbonswith other blowing agents which are not believed to cause suchenvironmental damage. Other blowing agents contemplated includehydrocarbons.

The general use of hydrocarbons including aliphatic and alicyclicalkanes as blowing agent when preparing polyurethane foam is known. U.S.Pat. No. 5,182,309 and European patent application EP-A-472,080 disclosethe use of aliphatic alkanes including pentane as polyurethane blowingagent. U.S. Pat. No. 3,586,651 discloses the use of acyclopentane/acetone mixture as blowing agent when preparing rigid,closed celled, polyurethane foam. Similarly in U.S. Pat. Nos. 5,096,933and 5,034,424 the use of cyclopentane and/or cyclohexane in combinationwith other blowing agents to prepare polyurethane foam is disclosed.Combinations of halocarbons and hydrocarbons as polyurethane blowingagent are also disclosed in U.S. Pat. Nos. 5,132,332 and 5,001,164.Whilst use of hydrocarbons as blowing agent provides vis-a-vis ozonedepletion a more environmentally acceptable system frequently, they donot permit the preparation of foam with long term commerciallyattractive physical properties including thermal insulation anddimensional stability characteristics.

Accordingly, it would be desirable to develop a process for thepreparation of polyurethane foam in the presence of a halogen-freeblowing agent and where the resulting foam exhibits long term thermalinsulation and dimensional stability characteristics better suited topresent commercial and environmental demands.

SUMMARY OF THE INVENTION

In one aspect, this invention is a process for preparing a polyurethaneor polyisocyanurate foam by reacting an organic polyisocyanate with apolyahl in the presence of a physical blowing agent characterized inthat the physical blowing agent, based on total amounts of a) and b),consists of:

a) from about 5 to about 80 mole percent of a C₅₋₆ alicyclic alkane ormixtures of such; and

b) from about 95 to about 20 mole percent of a mixture of isopentane andn-pentane present in a mole ratio of from about 80:20 to about 20:80.

In a second aspect, this invention is a process for preparing apolyurethane or polyisocyanurate foam by reacting components comprisingan organic polyisocyanate having an average of from about 2 to about 3.5isocyanate groups/molecule with a polyahl in the presence of water and aphysical blowing agent wherein, (a) the polyahl contains per 100 partsby weight of total polyahl:

a1) from about 30 to about 90 parts of an aromatic-initiated polyetherpolyol;

a2) from about 5 to about 35 parts of a liquid aromatic polyesterpolyol; and

a3) from about 5 to about 35 parts of an amine-initiated polyetherpolyol,

(b) the water is present in from about 1.5 to about 10 parts per 100parts by total weight of polyahl, and

(c) the physical blowing agent, based on total amounts of c1) and c2),consists of:

c1) from about 5 to about 80 mole percent of a C₅₋₆ alicyclic alkane ormixtures of such; and

c2) from about 95 to about 20 mole percent of isopentane and n-pentanepresent in a mole ratio of from about 80:20 to about 20:80.

In a third aspect, this invention relates to a polyurethane orpolyisocyanurate foam prepared by the process of this invention.

In a fourth aspect, this invention relates to an isocyanate-reactivecomposition for use in the preparation of polyurethane orpolyisocyanurate foam which comprises a polyahl, water, and a physicalblowing agent characterized in that:

a) the polyahl contains an aromatic-initiated polyether polyol presentin from 30 to 90 parts per 100 parts by total weight of polyahl;

b) the water is present in from about 1.5 to about 10 parts per 100parts by total weight of polyahl; and

c) the physical blowing agent, based on total amounts of c1) and c2),consists of:

c1) from about 5 to about 80 mole percent of a C₅₋₆ alicyclic alkane ormixtures of such; and

c2) from about 95 to about 20 mole percent of a mixture of isopentaneand n-pentane present in a mole ratio of from about 80:20 to about20:80.

In a fifth aspect, this invention relates to a hydrocarbon compositionsuitable for use as a physical blowing agent when preparing apolyurethane or polyisocyanurate foam which, based on total amounts ofa) and b), consists of:

a) from about 5 to about 80 mole percent of a C₅₋₆ alicyclic alkane ormixtures of such; and

b) from about 95 to about 20 mole percent of a mixture of isopentane andn-pentane present in a mole ratio of from about 80:20 to about 20:80.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the change with time of the thermal conductivity ofpolyurethane foam prepared in the presence of, with reference to theexamples later described, different hydrocarbon blowing agent mixtures.Foam 10 is illustrative of the present invention, Foams 1, 6 and 21 arecomparative foams.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, a hydrocarbon mixture is used as physical blowingagent to prepare a polyurethane or polyisocyanurate foam. The blowingagent consists of, as first component, an alicyclic C₅₋₆ alkane ormixtures of such; and as second component, a mixture of isopentane withn-pentane in a mole ratio of from about 80:20 to about 20:80. Theblowing agent contains, based on total amount of first and secondcomponent present, the first component in from about 5 to about 80 molepercent; and the second component in from about 95 to about 20 molepercent. When it is intended to use the resulting foam in applianceapplications, for reasons of long term thermal insulation performanceand dimensional stability, it is found advantageous to have theisopentane and n-pentane isomers present in a mole ratio of from about80:20 to about 60:40. When it is intended to use the resulting foam inconstruction applications, having different performance requirements toan appliance application, it is found advantageous to have theisopentane and n-pentane isomers present in a mole ratio of from about40:60 to about 20:80.

As first component, suitable C₅₋₆ alicyclic alkanes includecyclopentane, methylcyclopentane, cyclohexane, or mixtures of two ormore of these. Preferred is cyclopentane, and especially cyclohexane dueto its lower gas thermal conductivity. When the first component iscyclopentane advantageously, for reasons or boiling point and partialpressure, it is present in from about 5 to about 80, preferably in fromabout 15 to about 60, and more preferably in from about 20 to about 45mole percent. Conversely, the second component advantageously is presentin from about 95 to about 20, preferably from about 85 to about 40, andmore preferably from about 80 to about 55 mole percent. When the firstcomponent is methylcyclopentane or cyclohexane, or mixtures containing amajority of either of these alkanes, advantageously it is present infrom about 5 to about 40, preferably in from about 10 to about 35, andmore preferably in from about 20 to 35 mole percent. Conversely, thesecond component advantageously is present in from about 95 to about 60,preferably from about 90 to about 65, and more preferably from about 80to about 65 mole percent.

The hydrocarbon mixture of this invention is useful in preparingcellular polymers such as, for example, polyisocyanate-based foamincluding microcellular elastomers and especially rigid, polyurethaneand polyisocyanurate foams. In preparing such foam, an organicpolyisocyanate is reacted with a polyahl in the presence of thehydrocarbon mixture, and optionally water. It is often convenient topreblend the mixture with the polyahl before contacting same with thepolyisocyanate. It is, however, possible to simultaneously blend thepolyisocyanate, polyahl and hydrocarbon mixture in the production offoam. In making cellular polymers, it is advantageous to employsufficient blowing agent to provide a polymer having an overall densityfrom about 10 to about 700, preferably about 15 to about 300, and morepreferably about 20 to about 100 kg/m³. To provide for such density,typically the hydrocarbon mixture is present in an amount of from about1 to about 25, preferably from about 3 to about 22, and more preferablyin from about 10 to about 20 parts per 100 parts by weight of polyahl.

Polyahls which are useful in the preparation of the polyisocyanate-basedcellular polymers include those materials having two or more groupscontaining an active hydrogen atom capable of undergoing reaction withan isocyanate. Preferred among such compounds are materials having atleast two hydroxyl, primary or secondary amine, carboxylic acid, orthiol groups per molecule. Polyols, i.e., compounds having at least twohydroxyl groups per molecule, are especially preferred due to theirdesirable reactivity with polyisocyanates. Typically polyahls suitablefor preparing rigid polyurethanes include those having an equivalentweight of about 50 to about 700, preferably about 70 to about 300 andmore preferably about 70 to about 150. Such polyahls also advantageouslyhave a functionality of at least 2, preferably about 3, and up to about16, preferably up to about 8, active hydrogen atoms per molecule.Representative of polyahls include polyether polyols, polyester polyols,polyhydroxy-terminated acetal resins, hydroxyl-terminated amines andpolyamines. Examples of these and other suitable isocyanate-reactivematerials are described more fully in U.S. Pat. No. 4,394,491,particularly in columns 3 to 5. Preferred for preparing rigid foams, onthe basis of performance, availability and cost, is a polyol prepared byadding an alkylene oxide to an initiator having from about 2 to about 8,preferably about 3 to about 8 active hydrogen atoms. Exemplary suchpolyols include those commercially available under the trademarkdesignation VORANOL including VORANOL 202, VORANOL 360, VORANOL 370,VORANOL 446, VORANOL 490, VORANOL 575, VORANOL 800, all sold by The DowChemical Company, and PLURACOL 824, sold by BASF Wyandotte. Other highlypreferred polyols include alkylene oxide derivatives of Mannichcondensates, as disclosed, for example, in U.S. Pat. Nos. 3,297,597;4,137,265 and 4,383,102; and aminoalkylpiperazine-initiated polyethersas described in U.S. Pat. Nos. 4,704,410 and 4,704,411.

In a preferred embodiment of this invention, the polyahl is acomposition comprising an aromatic-initiated polyether polyol in fromabout 30 to about 90 parts per 100 parts by total weight of the polyahlcomposition. Further to the aromatic-initiated polyether polyol, thecomposition may also comprise an amine-initiated polyether polyol infrom about 5 to about 35 parts per 100 parts by total weight of thecomposition; or alternatively an aromatic polyester polyol, liquid atroom temperature, in from about 5 to about 35 parts per 100 parts bytotal weight of the composition. In a highly preferred embodiment ofthis invention, the polyahl composition contains the aromatic-initiatedpolyether polyol in from about 30 to about 90 parts; the amine-initiatedpolyether polyol in from about 5 to about 35 parts ; and the aromaticpolyester polyol in from about 5 to about 35 parts per 100 parts bytotal weight of the composition. Advantageously, the aromatic-initiatedpolyether polyol is an alkylene oxide adduct of a phenol/formaldehyderesin, frequently called a "novolac" polyol, such as disclosed in U.S.Pat. Nos. 3,470,118 and 4,046,721, or an alkylene oxide adduct ofphenol/formaldehyde/alkanolamine resin, frequently called a "Mannich"polyol such as disclosed in U.S. Pat. Nos. 4,883,826; 4,939,182; and5,120,815.

Polyisocyanates useful in making polyurethanes include aliphatic andcycloaliphatic and preferably aromatic polyisocyanates or combinationsthereof, advantageously having an average of from about 2 to about 3.5,and preferably from about 2.4 to about 3.2 isocyanate groups permolecule. Representative of suitable polyisocyanates include m- orp-phenylene diisocyanate, 2,4- or 2,6- toluene diisocyanate,hexamethylene-1,6-diisocyanate, tetramethylene-1,4-diisocyanate,cyclohexane-1,4-diisocyanate, hexahydrotoluene diisocyanate,naphthylene-1,5-diisocyanate, 1-methylphenyl-2,4-phenyldiisocyanate,diphenylmethane-4,4'-diisocyanate, diphenylmethane-2,4'-diisocyanate,4,4'-biphenylenediisocyanate,3,3'-dimethoxy-4,4'-biphenylenediisocyanate and3,3'-dimethyldiphenylpropane-4,4'-diisocyanate; triisocyanates such astoluene-2,4,6-triisocyanate and polyisocyanates such as4,4'-dimethyldiphenylmethane-2,2',5',5'-tetraisocyanate and the diversepolymethylene polyphenyl polyisocyanates. A crude polyisocyanate mayalso be used in the practice of this invention, such as the crudetoluene diisocyanate obtained by the phosgenation of a mixture oftoluene diamines or the crude diphenylmethane diisocyanate obtained bythe phosgenation of crude diphenylmethanediamine. Especially preferred,due to their ability to crosslink the polyurethane, aremethylene-bridged polyphenylpolyisocyanates. The ratio of equivalents ofisocyanates to equivalents of active hydrogen atoms is advantageouslyfrom about 0.5 to about 5, preferably about 0.9 to about 2, morepreferably about 1.0 to about 1.5. When the same ratio is expressed as amultiple of 100 this is then the isocyanate index.

In addition to the foregoing critical components, it is often desirableto employ certain other ingredients in preparing cellular polymers.Among these additional ingredients are water, catalysts, surfactants,flame retardants, preservatives, colorants, antioxidants, reinforcingagents, fillers, and the like.

Advantageously, water is present to supplement the blowing capacityalready provided by the hydrocarbon mixture. Water reacts withisocyanate leading the production of carbon dioxide. When present, thewater is preferably used in amounts of from about 1 to about 10,preferably from about 1.5 to about 8, and more preferably from about 2to about 6, and yet more preferably in from about 2.5 to about 5 partsper 100 parts by total weight of the polyahl.

Other auxiliaries useful when producing polyurethanes includesurfactants, pigments, colorants, fullers, fibers, antioxidants,catalysts, flame retardants, stabilizers and the like. In makingpolyurethane foam, it is generally highly preferred to employ a minoramount of a surfactant to stabilize the foaming reaction mixture untilit cures. Such surfactants advantageously comprise a liquid or solidorganosilicone surfactant. Other, less preferred surfactants includepolyethylene glycol ethers of long chain alcohols, tertiary amine oralkanolamine salts of long chain alkyl acid sulfate esters, alkylsulfonic esters and alkyl arylsulfonic acids. Such surfactants areemployed in amounts sufficient to stabilize the foaming reaction mixtureagainst collapse and the formation of large, uneven cells. Typically,about 0.2 to about 5 parts of the surfactant per 100 parts by weightpolyahl are sufficient for this purpose.

One or more catalysts for the reaction of the polyol (and water, ifpresent) with the polyisocyanate are advantageously used. Any suitableurethane catalyst may be used, including tertiary amine compounds andorganometallic compounds. Exemplary tertiary amine compounds includetriethylenediamine, N-methylmorpholine, N,N-dimethylcyclohexylamine,pentamethyldiethylenetriamine, tetramethylethylenediamine,1-methyl-4-dimethylaminoethylpiperazine,3-methoxy-N-dimethylpropylamine, N-ethylmorpholine, diethylethanolamine,N-cocomorpholine, N,N-dimethyl-N',N'-dimethyl isopropylpropylenediamine,N,N-diethyl-3-diethylaminopropylamine, dimethylbenzylamine and the like.Exemplary organometallic catalysts include organomercury, organolead,organoferric and organotin catalysts, with organotin catalysts beingpreferred among these. Suitable tin catalysts include stannous chloride,tin salts of carboxylic acids such as dibutyltin di-2-ethyl hexanoate,as well as other organometallic compounds such as are disclosed in U.S.Pat. No. 2,846,408. A catalyst for the trimerization of polyisocyanates,resulting in a polyisocyanurate, such as an alkali metal alkoxide mayalso optionally be employed herein. Such catalysts are used in an amountwhich measurably increases the rate of polyurethane or polyisocyanurateformation. Typical amounts are about 0.001 to about 1 parts of catalystper 100 parts by weight of polyahl.

In making a polyurethane foam, the polyol(s), polyisocyanate and othercomponents are contacted, thoroughly mixed and permitted to expand andcure into a cellular polymer. The particular mixing apparatus is notcritical, and various types of mixing head and spray apparatus areconveniently used. It is often convenient, but not necessary, topreblend certain of the raw materials prior to reacting thepolyisocyanate and active hydrogen-containing components. For example,it is often useful to blend the polyol(s), blowing agent, surfactants,catalysts and other components except for polyisocyanates, and thencontact this mixture with the polyisocyanate. Alternatively, allcomponents can be introduced individually to the mixing zone where thepolyisocyanate and polyol(s) are contacted. It is also possible toprereact all or a portion of the polyol(s), in the absence of water,with the polyisocyanate to form a prepolymer, although such is notpreferred.

The polyurethane foam of this invention is useful in a wide range ofapplications, such as in spray insulation, appliance foam, rigidinsulating boardstock, laminates, and many other types of rigid foam.

The following examples are given to illustrate the invention and are notintended to limit the scope thereof. Unless stated otherwise, all partsand percentages are given by weight.

EXAMPLE 1

Molded polyurethane foam is prepared by reacting, at an NCO index of1.25, VORANATE M220 a crude methylene diphenylisocyanate, average NCOfunctionality 2.7, available from The Dow Chemical Company with apolyahl composition in the presence of water, catalyst, and surfactantas detailed below. The components, at about 20° C., are mixed andstirred for 10 seconds at about 3000 rpm using a propeller mixer priorto pouring the reacting mixture into a mold having dimensions 30×30×10cm. Sufficient reacting mixture is poured into the mold to providemolded foam having an overall density of about 34 kg/m³. The hydrocarbonblowing agent composition used to prepare the individual foams alongwith the physical properties of the free-rise foam is reported in Table1.

    ______________________________________                                        78.7 pbw oxyethylene-oxypropylene adduct of a                                          phenol/formaldehyde resin, Hydroxyl                                           Number 196.                                                          26.2 pbw NIAX APP 315, an aromatic polyester                                           polyol with Hydroxyl No 315,                                                  available from Union Carbide Co.                                     26.2 pbw VORANOL RA640, an oxypropylene adduct                                         of ethylene diamine, hydroxyl number                                          640, available from The Dow Chemical                                          Company.                                                             13 pbw   diethylene glycol                                                    1.8 pbw  water                                                                3.25 pbw TEGOSTAB B8408, a proprietary                                                 silicon-based surfactant available                                            from Th. Goldschmidt AG.                                             1.17 pbw dimethylcyclohexylamine                                              1.3 pbw  CURITHANE 206, a proprietary urethane                                         catalyst available from The Dow                                               Chemical Company.                                                    ______________________________________                                    

Where reported the physical properties of the foams are observedaccording to the following test procedure for; core density, DIN 53420;thermal conductivity, ISO 2581; compressive strength, DIN 53421; anddimensional stability ISO 2796. When reported the average cell size,diameter, is determined from inspection of a thin section using apolarized-light optical microscope in combination with a Quantimet 520Image Analysis system. The accuracy of the measurement is considered tobe ±0.02 mm (20 micrometers).

                                      TABLE 1                                     __________________________________________________________________________               1*  2*  3*   4*  5*   6*    7*   8   9    10  11*                  __________________________________________________________________________    Blowing Agent                                                                 Cyclopentane (pbw)                                                                       0   2.93                                                                              5.86 8.8 11.73                                                                              14.66 0    2.93                                                                              5.86 8.8 11.73                (mole %)    (0%)                                                                             (21%)                                                                             (41%)                                                                              (61%)                                                                             (81%)                                                                               (100%)                                                                              (0%)                                                                              (21%)                                                                             (41%)                                                                              (60%)                                                                             (81%)                Pentane (pbw)                                                                            14.25                                                                             11.4                                                                              8.55 5.7 2.85 0     14.25                                                                              11.4                                                                              8.55 5.7 2.85                 (mole %)   (100%)                                                                            (79%)                                                                             (59%)                                                                              (39%)                                                                             (19%)                                                                              (0%)  (100%)                                                                             (79%)                                                                             (59%)                                                                              (40%)                                                                             (19%)                Ratio                                                                         iso-:n-pentane                                                                           0:100                                                                             0:100                                                                             0:100                                                                              0:100                                                                             0:100                                                                              /     20:80                                                                              25:75                                                                             33:66                                                                              50:50                                                                             100:0                Molded density                                                                           34.3                                                                              34.3                                                                              34.7 34.9                                                                              33.9 33.8  34.1 34.3                                                                              33.7 33.8                                                                              34.9                 (kg/m.sup.3)                                                                  Thermal                                                                       Conductivity,                                                                 parallel to rise                                                              (mW/m.K):                                                                     a) Initial 21.7                                                                              20.8                                                                              21.1 20.4                                                                              19.8 19.7  21.0 20.9                                                                              20.6 20.3                                                                              20.4                 b) after 58 days                                                                         24.6                                                                              23.8                                                                              24.2 23.7                                                                              22.8 23.0  24.0 23.8                                                                              23.3 23.1                                                                              23.8                   % change 13.3                                                                              14.4                                                                              14.7 16.2                                                                              15.2 16.8  14.3 13.9                                                                              13.1 13.8                                                                              16.7                 Cell Size (microns)                                                                      374 372 400  387 379  378   373  372 376  383 373                  Dimensional                                                                   Stability at 0° C.:                                                    a) 1 day   0   -2.0                                                                              -1.4 -0.2                                                                              -0.9 -14.0 -0.2 -2.0                                                                              -1.3 -1.0                                                                              -6.2                 b) 57 days -1.5                                                                              -6.6                                                                              -15.7                                                                              -0.4                                                                              -12.4                                                                              -24.6 -24.6                                                                              -3.1                                                                              -11.6                                                                              -3.0                                                                              -23.1                __________________________________________________________________________                   12* 13   14  15*  16*   17   18* 19*  20* 21*                  __________________________________________________________________________    Blowing Agent                                                                 Cyclopentane (pbw)                                                                           0   2.93 5.86                                                                              8.8  0     2.93 5.86                                                                              0    2.93                                                                              0                    (mole %)        (0%)                                                                             (21%)                                                                              (41%)                                                                             (61%)                                                                               (0%) (21%)                                                                              (41%)                                                                              (0%)                                                                              (21%)                                                                              (0%)                Pentane (pbw)  14.25                                                                             11.4 8.55                                                                              5.7  14.25 11.4 8.55                                                                              14.25                                                                              11.4                                                                              14.25                (mole %)       (100%)                                                                            (79%)                                                                              (59%)                                                                             (39%)                                                                              (100%)                                                                               (79%)                                                                             (59%)                                                                             (100%)                                                                             (79%)                                                                             (100%)               Ratio                                                                         iso-:n-pentane 40:60                                                                             50:50                                                                              66:33                                                                             100:0                                                                              60:40 75:25                                                                              100:0                                                                             80:20                                                                              100:0                                                                             100:0                Molded density 34.2                                                                              34.4 33.8                                                                              34   34.4  33.9 33.7                                                                              34.3 34.4                                                                              33.9                 (kg/m.sup.3)                                                                  Thermal                                                                       Conductivity,                                                                 parallel to rise                                                              (mW/m.K):                                                                     a) Initial     21.4                                                                              20.8 20.4                                                                              20.8 21.6  21.4 20.7                                                                              21.5 20.9                                                                              21.2                 b) after 58 days                                                                             24.0                                                                              23.8 23.3                                                                              23.5 24.4  24.0 23.5                                                                              24.2 23.5                                                                              23.9                 % change       12.1                                                                              14.4 14.2                                                                              13   13    12.1 13.5                                                                              12.5 12.4                                                                              12.7                 Cell Size (microns)                                                                          397 353  381 392  370   369  390 370  369 386                  Dimensional                                                                   Stability:                                                                    a) 1 day       -0.6                                                                              -2.7 -10.4                                                                             -0.8 -0.6  -0.8 -6.6                                                                              -1.7 -0.6                                                                              0                    b) 57 days     -2.3                                                                              -17.1                                                                              -24.0                                                                             -2.0 -1.5  -2.2 -21.9                                                                             -11.4                                                                              -1.1                                                                              -1.1                 __________________________________________________________________________     *Not an example of this invention                                        

When the thermal conductivity performance of Foam 10 and ComparativeFoams 1, 6 and 21 is followed for an extended period of time, theadvantageous long term thermal insulation performance of foams preparedwith the herein defined hydrocarbon blowing agent mixture becomesapparent. Foam 6 starts to exhibit a minimum loss of thermal insulationperformance relative to its initial value and when compared to the otherfoams; and to exhibit a minimum thermal conductivity value compared tothe other foams. FIG. 1 shows the advantageous thermal insulationproperties of Foam 6, over the other foams, after aging for 104 days.

    ______________________________________                                        Thermal Conductivity (mW/m.K)                                                 Days      Foam 1   Foam 6    Foam 10 Foam 21                                  ______________________________________                                         1        21.7     19.7      20.3    21.2                                      58       24.6     23.0      23.1    23.9                                     104       25.2     23.9      23.8    24.6                                     Difference:                                                                             3.5      4.2       3.5     3.4                                      after 104 days                                                                Dimensional                                                                             -1.5     -24.6     -3.0    -1.1                                     Stability:                                                                    (57 days)                                                                     ______________________________________                                    

These observations along with the reported dimensionally stabilityperformance indicate that polyurethane foam prepared according to thisinvention is superior to that of the art.

What is claimed is:
 1. An isocyanate-reactive composition for use in thepreparation of polyurethane or polyisocyanurate foam which comprises apolyahl, water, and a physical blowing agent characterized in that:a)the polyahl contains an aromatic-initiated polyether polyol present infrom 30 to 90 parts per 100 parts by total weight of polyahl; b) thewater is present in from about 1.5 to about 10 parts per 100 parts bytotal weight of polyahl; and c) the physical blowing agent consists,based on total amounts of c1) and c2), of: c1) from about 5 to about 80mole percent of a C₅₋₆ alicyclic alkane or mixtures of such; and c2)from about 95 to about 20 mole percent of isopentane and n-pentanepresent in a mole ratio of from about 80:20 to about 20:80.